BPSG films are widely used in the semiconductor industry, offering an insulating material with two key advantages: (1) BPSG can be flowed at a lower temperature than other materials to smooth steps and fill trenches, providing a gradually tapered contour that permits deposition of a uniform thickness of metal; and (2) the phosphorous in BPSG is capable of mobile ion gettering.
The ability to process at low temperature is increasingly important for the fabrication of integrated circuits with devices having short channel lengths and shallow junctions. By taking advantage of the ability to flow the glass at low temperature, a smoother surface is achieved without redistribution of dopants in these underlying integrated circuit devices. A surface free of sharply changing topology permits metal layers to be deposited with more uniform metal thickness and provides a more reliable insulation between conductive layers. Without a material that flows, it was found that the insulator and the conductor are much thinner adjacent steps, and these are the regions that are later found to produce either opens in the metal lines or short circuits between levels of metal. A BPSG anneal step that flows the glass eliminates the thinning problems and increases chip reliability.
Chemical vapor deposition (CVD) has long been used to deposit BPSG on integrated circuits. Three processes are available, atmospheric pressure chemical vapor deposition (APCVD), low pressure chemical vapor deposition (LPCVD), and plasma enhanced chemical vapor deposition (PECVD).
In the APCVD process used in manufacturing, wafers move on a belt through a furnace at elevated temperature under injectors that provide separate flows of reactant gases that mix and react on the wafer surface. Reactant gases typically emanate from different parts of the injector and reactants are held apart from each other for a short distance in the furnace by nitrogen blankets so that reactants mix and react only when they reach the wafer surface. Gases such as silane, oxygen, diborane, and phosphene are used as reactants. The process provides BPSG films that flow at low temperature. In particular, the BPSG films formed by APCVD flow at temperatures below 1000.degree. C. in a non-oxidizing environment.
However, the APCVD process poses significant manufacturing difficulties, including: (1) a high level of particulates on each wafer; (2) film thickness and film compositional variation within a wafer and between wafers; (3) the requirement for frequent injector replacement for cleaning, and the additional substantial tool down-time for requisite performance tests after injector replacement; and (4) the inability to quickly switch from BPSG deposition to another material deposition without changing injectors.
A process providing fewer defects, improved thickness and compositional uniformity, self cleaning of reactor parts, and the ability to switch the materials being deposited without changing reactor parts would therefore provide significant advantages to a manufacturer.
A process for depositing BPSG using PECVD has posed significant difficulties as reported by Tong et. al. in "Solid State Technology," January, 1984, p. 161-170 and by Kern and Smeltzer in "Solid State Technology," June, 1985, p. 171-179. One of the difficulties noted was that the PECVD film needed a 70.degree. higher anneal temperature to achieve the same wall angle when the anneal was conducted in a dry nitrogen environment than when it was conducted in a steam environment. The article also points out that annealing in a steam environment can cause problems for exposed contact openings, namely the growth of oxide in the contacts.
Thus, a method is needed to provide BPSG that retains the flow properties of the APCVD process without the disadvantages.